We sampled
the green crabs at sites where it was introduced (West
Coast USA, East Coast USA, Tasmania) and in sites where
it is native (Europe). Crabs achieved larger sizes
in the introduced regions, suggesting that he crabs performed
better ecologically. A comparison of limb loss among
regions indicated no difference in the rise of predation
between introduced and native regions. The presence
of nemertean worms and parasitism by larval trematodes,
acanthocephalans. Parasitic barnacles, on the other hand
was substantially higher in Europe than in introduced
regions. Trematodes and acanthocephalans were found in
Europe and on the East Coast of the USA while nemerteans
were found in Europe and on the West Coast of the USA.
Rhizocephalan barnacles were only found in Europe.
On the west coast, the nemertean worm transferred to the
green crab from a native shore crab. We experimentally
determined that this nemertean was capable of feeding
on ovigerous green crabs in the field. In modeling
the effect of a parasitic castrator, we found a direct
association between the prevalence of a parasitic castrator
and the degree to which the host population is reduced.
This association was only direct at larger spatial scales
where host recruitment was effectively closed. The parasitic
castrator was limited to host populations with a minimum
density. In some cases, time lags in the system led to
extinction of the host population. Addition of parasites,
as might be used in a biological control program also
led to extinction of the host population. This might be
augmented with a fishery for the host.